1. Field
The present invention relates generally to magnetoresistive devices and, more particularly, to magnetoresistive devices for magnetic random access memory applications.
2. Related Art
The discovery of the giant magnetoresistive (GMR) effect and the magnetic tunneling junction (MTJ) effect have led to the development of a number of spin-based electronic devices.
The GMR and MTJ effects are observed in certain thin-film devices that are made up of alternating ferromagnetic and nonmagnetic layers. The resistance of a typical device is lowest when the magnetic moments of the ferromagnetic layers are in a parallel orientation and highest when the magnetic moments are in an antiparallel orientation.
One type of GMR device is commonly referred to as a “spin valve.” A spin valve typically includes two ferromagnetic layers that are separated by a thin layer of a non-magnetic metal (usually copper) and also includes an antiferromagnetic layer that “pins” the magnetization of one of the ferromagnetic layers. FIG. 1 illustrates (in a simplified form) the layers in a typical spin valve 10. As shown in FIG. 1, spin valve 10 includes ferromagnetic layers 12 and 14 separated by a nonmagnetic layer 16. Ferromagnetic layer 14 is adjacent to an anti-ferromagnetic layer 18, such that the magnetization of ferromagnetic layer 14 is “pinned” in a particular orientation. The arrow in layer 14 indicates an exemplary pinned orientation, though, in general, the orientation could be pinned in either direction. Thus, the magnetization of ferromagnetic layer 14 remains relatively fixed when moderate magnetic fields are applied to spin valve 10. In contrast, the magnetization of ferromagnetic layer 12 is free to switch between parallel and antiparallel orientations, as indicated by the double-arrow symbol in layer 12. Thus, by applying an appropriate magnetic field to spin valve 10, the magnetization of ferromagnetic layer 12 can be changed while the magnetization of ferromagnetic layer 14 remains the same. In this way, applied magnetic fields can change the relative orientations of the magnetizations in ferromagnetic layers 12 and 14, which, in turn, can be detected as a change in resistance across the device. In particular, the resistance of spin valve 10 is typically lowest when the magnetizations of ferromagnetic layers 12 and 14 are parallel and highest when the magnetizations are antiparallel. For clarity, a first direction being antiparallel to a second direction indicates that the first direction is rotated 180 degrees from the second direction. During a read sequence, a read current (i) is passed across the spin valve as shown. Because the read current (i) flows parallel to the layers, a spin valve is known as a current-in-plane (CIP) device.
The layers of the GMR device may be formed using various techniques, including, for example, ion beam deposition, sputtering, plasma vapor deposition, evaporation, and/or molecular beam epitaxy.
The pseudo spin valve (PSV) is a second memory technology that uses the magnetoresistive effect to store data in a nonvolatile form. Like a spin valve, a pseudo spin valve typically includes two ferromagnetic layers that are separated by a layer of a nonmagnetic metal. The basic structure of the PSV is shown in FIG. 2 as a tri-layer device having a conducting spacer layer 26 separating a magnetic sense layer 22 from a magnetic storage layer 24. Each of the two magnetic layers has an associated magnetization direction along an easy axis of the PSV. The logical state of the PSV is determined by the magnetization direction of the reference layer.
In the PSV, the sense layer 22 is configured to switch its magnetization direction in response to the application of a magnetic field of at least a first threshold. Likewise, the storage layer 24 will switch its magnetization direction in response to the application of a magnetic field of at least a second threshold. The second threshold is generally higher than the first threshold—thus the magnetization direction of the sense layer 22 is easier to switch than the magnetization direction of the storage layer 24. Because the storage layer 24 has a higher switching threshold than the sense layer 22, the storage layer 24 is said to have a higher coercivity than sense layer 22. A double headed arrow at both sense layer 22 and storage layer 24 indicate that the respective magnetization directions of the layers may be inverted by an applied magnetic field.
The logical state of the PSV is determined by the magnetization direction of the storage layer 24. Thus, during a write sequence, a magnetic field of at least the second threshold must be applied to the PSV in order to switch its logical state. During a read sequence, a read current (i) is passed across the PSV. Because the read current (i) flows parallel to the layers, a PSV is also a CIP device.
Like GMR devices, MTJ devices typically include two ferromagnetic layers. In MTJ devices, the two ferromagnetic layers are separated by a thin tunneling barrier. As shown in FIG. 3, MTJ 30 includes ferromagnetic layers 32 and 34 separated by a non-magnetic, non-conducting barrier layer 36. Ferromagnetic layer 34 is configured such that its magnetization is “pinned” to a particular orientation. The arrow in layer 34 indicates an exemplary pinned orientation, though, in general, the orientation could be pinned in either direction. Thus, the magnetization of ferromagnetic layer 34 remains relatively fixed when moderate magnetic fields are applied to the MTJ 30. In contrast, the magnetization of ferromagnetic layer 32 is free to switch between parallel and antiparallel orientations relative to the fixed layer 34, as indicated by the double-arrow symbol in layer 32. Thus, by applying an appropriate magnetic field to MTJ 30, the magnetization of ferromagnetic layer 32 can be changed while the magnetization of ferromagnetic layer 34 remains the same. In this way, applied magnetic fields can change the relative orientations of the magnetizations in ferromagnetic layers 32 and 34, which, in turn, can be detected as a change in resistance. In particular, the resistance across MTJ 30 is typically lowest when the magnetizations of ferromagnetic layers 32 and 34 are parallel and highest when the magnetizations are antiparallel.
Usually, the resistivity of an MTJ is determined by measuring a read current passed perpendicularly through each layer of the MTJ. A read current (i) is shown passing perpendicularly through the layers of FIG. 3. Because of the direction of read current flow, an MTJ is termed a current perpendicular to plane (CPP) device.
GMR devices, including spin valves and pseudo spin valves, as well as MTJ devices, can be used as data storage elements in magnetic random access memory (MRAM) devices. In this regard, exemplary MRAM applications of GMR devices are described in U.S. Pat. Nos. 6,147,922; 6,175,525; 6,178,111; and 6,493,258, all of which are incorporated herein by reference. In typical MRAM devices, the logical state of a magnetoresistive memory element is based on its resistance, which, in turn, is based on the relative orientations of the magnetizations of the ferromagnetic layers. Thus, in one logical state, e.g., a “0” state, a magnetoresistive device may have its ferromagnetic layers in a parallel orientation and, thus, may exhibit a low electrical resistance. In the other logical state, e.g., a “1” state, the magnetoresistive device may align its ferromagnetic layers in an antiparallel orientation and, thus, may exhibit a higher electrical resistance. Data may be written to a magnetoresistive memory element by applying a magnetic field sufficient to change the magnetization of the “free” ferromagnetic layer, i.e., ferromagnetic layer 12 in spin valve 10 and ferromagnetic layer 32 in MTJ 30. In this way, the “free” ferromagnetic layer functions as a “switching layer” that stores data in the form of a particular magnetization orientation relative to the other ferromagnetic layer, the “reference layer.” Thus, in spin valve 10, ferromagnetic layer 12 may function as the switching layer, and ferromagnetic layer 14 may function as the reference layer.
The magnetic fields used to write data to a magnetoresistive memory element in an MRAM device are typically generated by a “word” current flowing in a nearby conductor. For example, a word current flowing in one direction may be used to place the magnetoresistive memory element in one logical state, and a word current flowing in the other direction may be used to place the magnetoresistive memory element in the other logical state. In a further embodiment, during a write cycle, a second current may additionally be passed through another conductor aligned near the memory element such as a sense line or a bit line, for example. The second current creates a second magnetic field. The second magnetic field generally has an additive effect on the magnetic field created by the word current and further enables switching the logical state of the magnetoresistive memory element.
In addition, for redundancy and better signal sensing, some of the common MRAM cells include two magnetoresistive elements that are in opposite logical states. Thus, to change the state of the two magnetoresistive devices in the memory element, the word current is often arranged to apply magnetic fields of the same magnitude but opposite sign to the two magnetoresistive devices. Because of magnetic hysteresis, the switching layer may retain its magnetization orientation relative to the reference layer even when the word current stops and the magnetic field that the current generated is no longer present. In this way, little or no power may be needed in order for a magnetoresistive memory element to retain its logical state. Accordingly, MRAM devices are generally regarded to be a form of non-volatile data storage.
One difficulty with conventional GMR and MTJ devices for MRAM applications, e.g., for write and/or read operations, depending on the architecture, is that the hysteresis curve for the device is often substantially asymmetric or biased with respect to an applied magnetic field.
FIG. 4 illustrates such an asymmetric hysteresis curve. In FIG. 4, the vertical axis represents the resistance of an exemplary magnetoresistive device, and the horizontal axis represents an applied magnetic field. This magnetoresistive device exhibits a resistance R1 in zero applied magnetic field, after a magnetic field H1 is applied, and exhibits a resistance R0 in zero applied magnetic field, after a magnetic field H0 is applied. Thus, R1 may represent the resistance of the GMR device in the “1” state, and R0 may represent the resistance of the GMR in the “0” state.
Several disadvantages may result from this asymmetric hysteresis curve. First, because the hysteresis curve is not centered about zero applied magnetic field, the difference between the two zero-field resistances, R1 and R0, may be much smaller than the maximum possible resistance difference possible that the GMR device can exhibit. Second, the asymmetry of the hysteresis curve may cause higher word currents to be required for reliable operation. In particular, since word currents of the same magnitude but different directions are typically used to write data to the GMR devices in an MRAM memory element, a word current that generates an applied magnetic field with magnitude H0 may be insufficient. Flowing in one direction, the word current may be able to place the GMR device in the “0” state with resistance R0. However, when flowing in the other direction, the word current may be unable to place the GMR device in the “1” state with resistance R1. Instead, a higher word current, sufficient to generate an applied magnetic field of magnitude H1 may be required for reliable operation.
Accordingly, there is a need for magnetoresistive devices that exhibit hysteresis characteristics that are more compatible with MRAM applications.